In a typical cryostat retaining a body of liquid cryogen, heat leaking in from the ambient environment is removed by boil-off of the cryogen. Generally, the cryostat has an outer housing, an inner container for the liquid cryogen, a transfer channel from the outer housing to the inner container and a radiation shield surrounding the inner container and in thermal contact with the transfer channel. The boil-off travels up through the transfer channel from the inner container in heat exchange relation with the radiation shield. The boil-off absorbs heat from the radiation shield and is vented to ambient through an outer end of the transfer channel. The amount of heat removed from the cryostat by the boil-off is not limited to the heat of vaporization of the cryogen alone, but is the combination of the heat of vaporization and the sensible heat gain in the gaseous cryogen as it warms to ambient conditions. For the low boiling point gases of Ne, H.sub.2, He the sensible heat gain far outweighs the heat of vaporization.
If a recondenser is positioned in the transfer channel then the boil-off cooling of the cryostat must be replaced by the recondenser. Hence, the recondenser must extract the load associated with the lost sensible heat gain. This imposes a significantly higher heat load on the recondenser than one would calculate from the boil-off rate of the body of cryogen alone. A typical solution is to provide sufficient refrigeration at the boiling point temperature of the cryogen to handle the combined loads.
In a particular application to superconducting devices of today, a cryostat or vacuum jacketed reservoir of liquid cryogen is used to cool the device to achieve superconductivity. Typically the cryostat has a liquid cryogen boil-off rate of about 0.3 liters per hour. This equates to a heat leak of 0.212 watts to the liquid bath. When this boil-off is recondensed with a recondenser, the total heat leak to the liquid cryogen bath is over three watts which is an increase by a factor of fourteen. Accordingly in such superconducting devices and other applications employing a recondenser, there is a need for efficient management of heat leak into the cryostat.